Can Earth take the heat of 'global brightening'?

For the most part, "they're placed where people feel like taking measurements - near population centers - so they can be operated and serviced," says Ellsworth Dutton, a scientist with the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo. He took part in two of the three "brightening" studies.

Still, the fact that three groups spot the same trend, however tenuous, gives them confidence that they are seeing something real. "I was quite skeptical of the dimming results when they first came out," Dr. Dutton says, based on the instruments and statistical techniques used. "But there's no instrument deficiency I can think of that would cause data to show an increasing trend" in solar radiation at the surface. "It's hard to get that unless it's real."

He adds that in his mind, the newest results bolster the case for the 30-year decline. "You can't keep going up without starting somewhere and you can't keep going down without turning back up."

What's causing the changes? Scientists are convinced the culprit lies within the atmosphere, rather than with the sun itself. No changes in the sun's output have yet been measured that would account for the measured changes in solar radiation striking the surface, they say.

Dutton, for one, says he suspects that scientists may be seeing an oscillation of some sort that could occur over several decades. Others, such as Charles Long of the Department of Energy's Pacific Northwest National Laboratory in Richland, Wash., suspect aerosols. "They'd be my first guess," he says.

Fossil fuels give off carbon dioxide, a greenhouse gas, when burned. But they also yield sulfur dioxide and oxides of nitrogen, which form tiny particles in the atmosphere. These particles, or aerosols, can reflect incoming radiation back to space and act as seeds around which water vapor can condense to form cloud droplets.

As if to underscore the link, scientists in China published a solar-radiation study in March covering 40 years of measurements there. During the second half of the 20th century, the country was burning enormous amounts of fossil fuels. The team noted "significant decreases" in solar radiation from 1965 into the late 1980s. Solar radiation at the Earth's surface fell by some 5.4 percent per decade. Then it began to pick up again, at least through 1994. The aerosol particles from burning fossil fuels are "likely to have contributed to the decrease in clear days in China," the team reports in the March 17 issue of Geophysical Research Letters.

"That's a nice addition to the growing body of evidence" for the ties between so-called "global dimming" and man-made aerosols, says Lamont-Doherty's Dr. Liepert. Her studies suggest some potentially odd effects from a combination of global warming and aerosols. The result could be less sunshine and less rainfall, despite a warmer, more moist world. In modeling work published last May, she and colleagues at the Max Planck Institute for Meteorology in Hamburg and at Dalhousie University in Halifax, Nova Scotia, found hints that heat can get trapped between clouds and layers of aerosols, reducing the amount of radiation reaching the surface. Less heat at the surface translates into less evaporation to form clouds. Any clouds that do form have droplets too small to rain out, leaving the land below high and dry.

• More than 1 million Earths could fit inside the sun, which measures nearly 1 million miles across.

• With a surface temperature of about 10,000 degrees F., the sun pumps out a lot of energy. Every square meter at the top of Earth's atmosphere receives about 1,400 watts of energy. Some of that gets reflected back to space or absorbed in the atmosphere. The amount reaching Earth depends on latitude, weather, the season, and time of day.

• Solar panels convert that energy into electricity. But actually, most of our conventional energy also originated from solar power. Oil, natural gas, and coal came from decayed plants and animals that got their energy from photosynthesis.

Sources: The Complete Idiot's Guide to The Sun; Solarserver; Arizona State University

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